In our earlier U.S. Pat. No. 5,139,025, and in subsequent U.S. Pat. No. 5,217,013, assigned to the same assignee, various sensor configurations and structures are disclosed for use in spectrophotometric clinical examination apparatus, particularly the cerebral oxygen saturation monitor developed by Somanetics Corporation, of Troy, Mich., which uses electro-optical components mounted in such a sensor to emit light energy of selected wavelengths and project the same through brain tissue located behind the torehead by transmissivity through the epidermal layers and underlying bone of the frontal skull, and to detect resultant light energy at certain locations spaced laterally from the point of light introduction by certain predetermined distances. In the first such patent, a relatively rigid "hard" sensor configuration is disclosed which is principally suitable for use on generally flat or very soft, compliant surfaces and media, while in the second such patent a flexible, compliant sensor is disclosed which is suitable for use on various curved surfaces, to which it may be manually conformed, such as for example the human forehead.
As indicated above, the lateral distance between the light source and detectors used in such sensors is of considerable importance, since such distances in effect determine the depth to which the interrogating light spectra will penetrate the underlying physiological tissue, at least to the extent that sufficient resultant light is detectable by the sensors to allow for processing and analysis which will yield meaningful data as to the state, condition, or other such attributes of the internal tissue sought to be analyzed. Prior patentees have also referred to this principle, or effect, at least in one way or another; for example, F. Jobsis refers to this in his earlier U.S. Pat. No. 4,223,680, although he appears to primarily attribute the underlying principle or rationale to the belief (not shared by the present inventors) that the interrogating light spectra will traverse the scalp, skull, and "gray matter" of the brain immediately underlying the skull along a rectilinear path, but will be abruptly reflected along another such path by the "white matter" of the brain, with a small amount of the light being directed back to the source but most of it being deflected orthogonally and passing back out of the head through the skull and scalp, etc. a particular distance away from the source. In point of fact, Jobsis categorically asserts in one or more of his patents that an absolute minimum separation distance of 4.25 centimeters exists in all such cases, which must be observed if the "gray matter" is to be traversed by the examining spectra, and which will thus control the operation of all such devices.
As indicated, the present inventors do not share the opinion just stated, and on the contrary have demonstrated that other factors and principles are involved, and that the transmission of light energy of selected spectra through the brain will essentially exhibit the characteristics of transmission through a highly scattering and partially absorptive media, through which an essentially infinite number of randomly varying transmission paths will occur, all of which, as a general matter, defining a theoretical mean optical path which is arcuately curved, and in the simplest case, essentially a circular arc, between the source and any given detection location, with an exponential decrease in the intensity of the light as a function of the length of the path it has followed to any given point spaced laterally from the point of origin.
Further, the present inventors have previously disclosed the advantages of using two different detectors, or detector groupings, located at mutually different distances from the source of light energy, one being considered a "near" detector and the other a "far" detector, so that the optical response data produced by each could be comparatively analyzed and the effects upon the dam produced by the "far" detector (which samples light that has penetrated more deeply into the subject) can be conditioned so as to in effect eliminate from it tile optical response data which is attributable to the skin, bone, and related skeletal tissue and vascularity, etc., thereby producing data which effectively characterizes only the internal (e.g., brain) tissue. For the most part, however, it was previously thought that the "near" detector should be located in very close proximity to the source, for a variety of reasons. This view is also reflected in the aforementioned patents of Jobsis, at least certain of which also show the use of both a "near" and "far" detector in the same sensor, although the specific reasons for doing so are not considered to be very well, or clearly explained in these patents.